A Thai woman and New Zealand woman were “probably” exposed to a “toxic chemical, pesticides or gas”, it says.
The deaths of British couple George and Eileen Everitt were “possibly related” as they occurred in the same hotel.
The death of a US woman in another hotel is also linked to toxic exposure.
The deaths, including that of the Everitts from Lincolnshire, took place in the northern city of Chiang Mai in January and February. Several other tourists also fell ill.
A five-month investigation by Thai authorities with the help of the World Health Organization (WHO) followed but a report on Thailand’s Department of Disease Control on Tuesday said the exact toxic agents could not be identified.
But an investigative report by a New Zealand TV station reportedly found traces of a toxic insecticide – chlorpyrifos – used to kill bedbugs in the room where the New Zealand tourist had stayed.
A French woman who also died in January in Chiang Mai is said to have developed fever and other symptoms before arriving in the city, and so her case is probably not related, the report says.
Thailand is one of the world’s most popular tourist destinations, attracting about 14 million visitors every year.
The Department of Disease Control listed measures it said would be taken to prevent more illness, including monitoring pesticide and chemical use in hotels.